1. Field of the Invention
The present invention relates to a thickness shear mode quartz crystal element which may have a considerably compact shape and provide good vibration characteristics, and a method for manufacturing the same.
2. Description of the Prior Art
Recently, piezoelectric elements have been frequently used as a reference for frequency or time in a various kinds of electronic devices. Among the piezoelectric elements, a quartz crystal element with synthetic quartz has been widely used in many types of electronic devices since it has physically and electrically outstanding characteristics and can be supplied in a large quantity. As there have been increased needs for more compact and lighter electronic devices, a quartz crystal element has been needed to be more compact. A compact quartz blank for a quartz crystal element has been conventionally strip-shaped or circular.
Quartz crystal elements can be classified into AT-cut, BT-cut or the like according to a cutting direction of a quartz blank from a single crystal of quartz. Furthermore, according to the cutting direction of the quartz blank, quartz crystal elements can be classified into vibration modes such as thickness shear, stretching and tuning-fork modes. In particular, an AT-cut quartz crystal element may be easily miniaturized, provides a convenient resonance frequency and has a temperature coefficient of nearly zero, and thus it is most widely used. An AT-cut quartz crystal element operates in a thickness shear mode. As is well known, X, Y and Z axes are crystallographically defined for quartz. An AT-cut quartz blank is cut from an X-Z' plane which is taken by rotating an X-Z plane of a quartz crystal by about +35.degree. around the X axis of the crystal. In such a quartz blank, it is known that a direction rotated by about 28.degree. to the X axis from the Z' axis is an axis direction of zero stress sensitivity in which frequency change against stress impressed from outside become minimum. Specifically, AT-cut quartz crystal elements include one using a regular square crystal blank, one of whose diagonal lines is the above axis of zero stress sensitivity and which is held at the both ends of the diagonal line.
It is believed to be desirable that an area of a quartz blank of an AT-cut quartz crystal element is large for providing ideal vibration characteristics. In an AT-cut quartz crystal element, size reduction of a quartz blank for miniaturization may cause deterioration in vibration characteristics such as an unwanted response point with a frequency different from that of the principal resonance; marked reduction in Q (quality factor) value; and sudden jumping of the resonance frequency due to some temperature variation. Thus, excessive size reduction of a quartz blank may often lead to an impractical quartz crystal element.
Problems such as unwanted response and jumping of a resonance frequency are not limited to the AT-cut quartz crystal element, but may be observed in a quartz crystal element in general. The smaller the quartz blank is, the more difficult control of unwanted response is; the more readily jumping of the resonance frequency may be caused by temperature variation; and the higher the electric impedance is, causing it difficult to provide appropriate vibration. For a quartz crystal element with a strip-shaped quartz blank, size reduction of its longer edge to about 5 mm may make it quite difficult to provide good vibration characteristics and are liable to cause strong unwanted response. Therefore, in manufacturing a small size of quartz crystal element, it is necessary to find a quartz blank which can provide good vibration characteristics, by, for example, gradually changing the width of the quartz blank. It may require a large amount of time and labor.
Furthermore, a conventional quartz crystal element has problems in mounting during enclosing a quartz blank in a case. FIG. 1 is a perspective view of configuration of a conventional quartz crystal element where a quartz blank is enclosed in a metal case. A pair of lead wires 2 are implanted on a base 1 and mutually insulated. To the tips of the lead wires 2, holding members 3 which are made of ribbon-like metal sheets and face each other are adhered by, for example, welding. A slot 4 with a defined width is formed on each of the holding members 3 along the longitudinal direction the holding member 3.
A quartz blank 5 is formed as, for example, a circle and ground to a thickness depending on a desired resonance frequency. Excitation electrodes 6 are formed on the major surfaces of the quartz blank 5, respectively. The excitation electrodes 6 are extended in mutually opposite directions and led to the plate edges of the quartz blank 5. The leading ends are inserted into and then, with conductive adhesive 7, adhered to the slots 4 of the holding members 3. Thus, the quartz blank 5 is held by the holding members 3. Then, a case 8 is placed, which is then sealed on the base 1 with filling an inert gas thereto to provide a quartz crystal element.
The length held by the slots is shorter for a strip-shaped or square quartz blank than for a circular one. When the length held by the slots is short and the slot width is relatively wider to the thickness of the quartz blank, the quartz blank becomes indirectly held only by conductive adhesive. In such conditions, a quartz crystal element may exhibit poor vibration resistance, poor shock resistance and aged deterioration.
Thus, the quartz blank is preferably clamped in the slots and auxiliarily adhered to them with conductive adhesive to ensure mechanical holding and electric conduction, resulting in good vibration resistance, good shock resistance and small aged deterioration. It is, therefore, desirable that the width of the slot corresponds to the thickness of the quartz blank.
However, in a thickness shear quartz crystal element, the resonance frequency depends on the thickness of the quartz blank. Thus, if it is desired to use a holding member with a slot width corresponding to the thickness of the quartz blank, there should be prepared many holding members different in their slot width depending on a given resonance frequency, which is impractical.
Recently, surface mounting types of electronic parts have been increasingly used because of miniaturization of electronic devices and increasing automation of their assembling processes. Thus, more compact surface-mounting types of quartz crystal elements have come to be desired. There are, however, some problems to be solved for achieving compact surface-mounting quartz crystal elements.
Since a quartz crystal element utilizes piezoelectric vibration of quartz, a quartz blank should be held in space and placed in a chemically and physically stable atmosphere. The quartz blank should be, therefore, held in a case having an adequate volume and the case should be tightly sealed. However, it is now difficult to accurately place and hold a small quartz blank in a compact surface-mounting type of case and to ensure sealing of the case. Furthermore, for obtaining a surface-mounting type of a quartz crystal element, the size of the quartz blank should be considerably smaller than any of those of the prior art. In such a quartz blank, it is, however, difficult to provide good vibration characteristics, as described above.